The entire disclosure of Japanese Patent Application No. 2000-064427 filed on Mar. 9, 2000 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a thread forming tap and a threading method.
2. Description of the Related Art
As shown in FIG. 6, a conventional thread forming tap has a threaded portion 100 comprising a chamfer 101 which is tapered to form a complete mountain-like (angle) shape (see inclination angle in FIG. 6), and a parallel thread 102 which is formed continuously with the chamfer 101. The thread forming tap also comprises a plurality of oil grooves disposed in a circumferential direction of the thread forming tap.
When the thread forming tap is ground, as shown in FIG. 7, an end of the thread forming tap (raw material) W on the side of the threaded portion 100 and an end of the thread forming tap on the side of a shank 103 are supported by center portions 104a and 104b of a thread grinder, the thread forming tap W is rotated and in this state, the thread forming tap W is constantly fed to a rotating grinding stone 105 to carry out a traverse grinding.
In the conventional thread forming tap, since the chamfer 101 is tapered to form the complete mountain-like shape, the grinding stone 105 goes in and out from the chamfer 101 deeply (as compared with the parallel thread 102) correspondingly at the time of grinding.
Therefore, if the thread forming tap has a small diameter (like a miniature thread such as a hard disk), there is a problem that a tip end of the grinding stone 105 abuts against the threaded portion-side center portion 104a to damage the latter, and the chamfer 101 can not be formed.
Further, according to the conventional grinding method, since the thread forming tap is fed constantly, in the chamfer 101 immediately before it is fed to the parallel thread 102, a pitch of any of a crest and a root of the thread is formed differently.
Thus, at the time of tapping, in the chamfer 101 immediately before it is fed to the parallel thread 102, an excessive force is applied to a front or rear flank of the thread, there is a problem that an amount of heat is increased, and this shortens the life of the tap.
The present invention has been accomplished in view of the above circumstances, and it is an object of the invention to provide a thread forming tap to which a chamfer can easily be mounted, and a general threading method.
To achieve the above object, according to the present invention, there is provided a thread forming tap having a threaded portion comprising a tapered chamfer and a parallel threaded portion which is formed continuously with the chamfer, wherein a depth of root of the chamfer is set equal to that of a root of the parallel threaded portion constantly.
The invention also provides a threading method for a thread forming tap having a threaded portion comprising a tapered chamfer and a parallel threaded portion which is formed continuously with the chamfer, wherein after a parallel portion from the chamfer to the parallel threaded portion is continuously ground, a front flank to a rear flank of the chamfer are ground while alternately and gradually changing a feeding ratio between a tip end and a rear end of the chamfer and then, an outer diameter portion of the chamfer is ground.
The invention also provides a threading method wherein a front flank and a rear flank of a thread are formed while alternately and gradually changing a feeding ratio between a tip end and a rear end of threaded portion and then, a parallel threaded portion is formed and thereafter, an outer diameter portion of the thread is ground.